How can i do this?

I can be wrong, but I’m quite sure there’s nothing similar yet.
You can write or find someone who can write a plugin, and make it. It won’t be that difficult.

Ok i dont really want to go that far with things, how about a way of collecting everybodys email address that signs up for the rss feed, then I will randomly pick.

Are there any competition plugins for WP? A google search just returns the make a plugin competition for wordpress

Generally, subscribing to an RSS feed is a one-way process: The users feed aggregator gets the feed from your site without giving you any information (such as email addresses). You’re asking for something beyond what RSS is designed to do.

A better approach would be having them sign up for email digests, then randomly selecting a winner. That way, they actually have to give you their email address to get signed up. Look for the subscribe2 plugin. It doesn’t do the competition, but you can just look at the subscribers list and figure it out.

Tell them they must subscribe via e-mail and use the =rand() function in Excel. Look at the help to tell it to do whole numbers and to stay within your range. 1-# #=the last subscriber number.